Elastomeric compounds suitable for such applications as motor vehicle tire tread, for example, typically employ carbon black fillers as reinforcing agents to provide high abrasion resistance and good hysteresis properties, including low hysteresis at elevated temperatures (e.g., 70.degree. C.). Other applications employing elastomers exhibiting good abrasion and/or hysteresis properties include other tire components, such as undertread, wedge compounds, sidewall, carcass, apex, bead filler and wire skim, as well as engine mounts and base compounds used in industrial drive and automotive belts. In this regard, it is well-known, of course, that elastomers are not completely elastic, such that upon recovery from deformation only a part of the energy used to deform the elastomer is returned. The lost energy, hysteresis, usually manifests itself in the form of heat. This energy loss can be a significant disadvantage in applications such as tire tread, since it results in undesirable rolling resistance. Thus, the hysteresis of an elastomeric compound under cyclic deformation, such as the cyclic deformation experienced by a tire tread in normal usage, is the difference between the energy applied to deform the elastomeric composition and the energy released as the elastomeric composition recovers to its initial undeformed state.
Hysteresis is known to be well-characterized by a loss tangent, tan .delta., the ratio of the loss modulus to the storage modulus, that is, viscous modulus to elastic modulus. Also characterized as the ratio of energy lost to energy returned, the loss factor tan .delta. is widely used to indicate tire performance properties. Tan .delta. values of an elastomeric composition used in tire tread, measured at low temperatures (for example, -30.degree. C. to 0.degree. C.) are used as an indication of wet traction capability, with higher values being desirable. For rolling resistance, typically, measurement of tan .delta. may be based on a temperature in the range of 40.degree. C. to 70.degree. C., with lower values being desirable. However, the amplitude of deformation also has a significant effect on performance, so it is also known to test hysteresis over a strain sweep (corresponding to a range from low to high deformation amplitude) at one or more fixed temperatures. The highest value measured for a given temperature, tan .delta..sub.max, is an indicator of rolling resistance, with lower values of tan .delta..sub.max being desirable as corresponding to lower rolling resistance. Thus, tires made with a tire tread compound having lower hysteresis measured at higher temperatures, such as 40.degree. C. or higher, will have correspondingly lower rolling resistance, which in turn can result in reduced fuel consumption by a vehicle equipped with such tires. Desirably, however, such tire tread compound should also have high hysteresis at low temperature for good wet traction.
Particulate filler materials in addition to carbon black also are known for use in elastomeric compositions, including various grades of silica. Silica alone as a reinforcing agent for elastomer typically yields compositions having poor performance characteristics for tire applications, compared to the results obtained with carbon black alone as a reinforcing agent. It has been theorized that strong filler-filler interaction and poor filler-elastomer interaction may account, in part, for such performance properties of silica alone. The silica-elastomer interaction can be improved by chemically bonding the two with a silane coupling agent, such as bis(3-triethoxysilylpropyl) tetra-sulfane, commercially available as Si-69 from Degussa AG (Germany). Coupling agents such as product Si-69 are generally believed to create a chemical linkage between the elastomer and the silica, thereby coupling the silica to the elastomer. When the silica is chemically coupled to the elastomer, certain performance characteristics of the resulting elastomeric composition are enhanced. When incorporated into vehicle tires, certain such elastomeric compounds have been found to provide, for example, improved hysteresis balance. Unfortunately, silica fillers typically are more expensive than comparable carbon black fillers, resulting often in an undesirable cost penalty for their use in elastomeric compositions. In addition, silane coupling agents such as Si-69 are quite costly, further exacerbating the cost penalty.
Coupling agents suitable for silica fillers are discussed, for example, in F. Thurn and S. Wolff, Kautsch. Gummi Kunstst. 28, 733 (1975)). As noted there, such coupling agents are generally composed of a silane compound having a constituent component or moiety (the silane portion) capable of reacting with the silica surface and, also, a constituent component or moiety capable of reacting with the elastomer molecule, particularly a sulfur vulcanizable rubber having carbon-to-carbon double bonds or unsaturation. In this manner, the Thurn et al paper states that the coupling agent acts as a connecting bridge between the silica and the rubber and thereby enhances the rubber reinforcement performance of the silica filler. A report by the Malaysian Rubber Producers Research Association ("the MRPRA report"), Functionalization of Elastomers by Reactive Mixing, Research Disclosure--Jun. 1994 (p. 308) shows a vulcanized 60:40 natural rubber:EPDM elastomer blend comprising 50 phr N660 carbon black filler to have less bound rubber (g/g black) in the natural rubber portion and more in the EPDM portion when modified by reaction with chemicals currently employed in accelerated sulfur vulcanization of rubber compounds, including bis-4-(1,1-dimethylpropyl)phenoldisulfide ("BAPD") and dithiodimorpholine ("DTDM"). The use of dithiodicaprolactam ("DTDC") is shown to yield increased bound rubber in both. The modification by mixing at temperatures in excess of 150.degree. C. is said to yield improved properties in the ultimate vulcanizates. An increase is reported for both SBR and EPDM-1 through modification of the elastomer with dithiodicaprolactam during mixing of the elastomer with 50 phr N330 carbon black. Other additives have been suggested for use together with curatives or a vulcanization system, including 1,3-bis(citracon imidomethyl) benzene ("BCI-MX") sold under the trade name Perkalink 900, Akzo Nobel Chemicals, Inc., Akron, Ohio, USA. Such BCI-MX additive is said to serve as an antireversion agent during curing of a composition incorporating CBS, 6PPD, APDS, carbon black (N-375), aromatic oil (Dutrex 729 BP), zinc oxide, stearic acid and sulfur. The MRPRA report and another such report were characterized in Rubber Reviews (published by the Rubber Division, American Chemical Society) as showing modification of elastomers with sulfur donors by mixing at the elevated temperatures typical of the preparation of masterbatches in an internal mixer to achieve low levels of modification both in the absence and presence of carbon black during mixing. Such modification of the elastomers is analogized there to elastomer modification wherein a functional group (e.g., morpholine, caprolactam or alkyl phenol mono-sulfide) is bound to the rubber via a sulfur link, and this functional group is then later displaced, e.g., by 2-mercaptobenzothiazole ("MBT") etc., to create a crosslink precursor site on the rubber.
It is an object of the present invention to provide novel elastomer compositions having good abrasion and hysteresis properties. In accordance with certain preferred embodiments, it is an object to provide novel multi-stage processes for producing such elastomer compositions. Other objects and features of the invention will become apparent from the following disclosure.